The present invention relates to impedance cardiography. Impedance cardiography is a method of using the measured electrical impedance of the body to determine cardiac output.
When electrodes are connected at two locations on the human body, and an alternating electric current is made to flow through the body, from one electrode to the other, one finds that the body has a measurable impedance which varies with time. If the electrodes are placed such that the current flows through the thorax, the changes in impedance result from changes in the amount of blood flowing in the vessels in the region between the electrodes. In general, the effective impedance of the portion of the body between the electrodes varies inversely with the amount of blood in these vessels. Thus, in theory, one can determine the amount of blood in the thoracic vessels, at a given time, from a measurement of electrical impedance of the body. Such impedance is called "bioimpedance" because it comprises impedance of a set of biological tissues.
The instantaneous amount of blood in the vessels is directly related to the performance of the heart. When blood is pumped out of the heart, the vessels in the thorax become momentarily filled with blood, and the impedance in the thorax decreases rapidly. After the ventricular contraction is complete, the impedance increases to its former level. Analysis of bioimpedance can therefore provide information on cardiac output.
Other investigators have developed mathematical models which express cardiac output as a function of certain parameters which can be derived from measurements of bioimpedance. Examples of such models are found in Re. U.S. Patent No. 30,101 (Kubicek) and U.S. Pat. No. 4,450,527 (Sramek). The disclosures of both of the above-cited patents are incorporated by reference into this specification. The Sramek patent also describes another model published by Kubicek in 1974, after the issuance of the original Kubicek patent. Both the model disclosed by the Sramek patent, and the model developed by Kubicek in 1974 require measurement of two critical parameters to determine stroke volume (and hence cardiac output). The first parameter is the maximum excursion of the first derivative of the impedance signal, starting at a time corresponding to the opening of the aortic valve. The second parameter is the ventricular ejection time (VET), which is equivalent to the time interval between the opening and subsequent closing of the aortic valve.
The technique of using bioimpedance measurements to determine cardiac output has great allure, because it enables the physician to obtain important information on heart function with an entirely noninvasive procedure. Alternative methods of determining cardiac output require heart catheterization, which carries an inherent risk, and which is relatively expensive.
Unfortunately, the bioimpedance techniques of the prior art have not been very successful in reliably measuring cardiac output. The primary problem with bioimpedance measurements is separating the "true" signal from spurious signals, or "artifacts". For example, the breathing of a patient is known to affect the impedance profile of the body. Some investigators have suggested taking impedance measurements while the subject briefly stops breathing. Obviously, the latter solution is not desirable and not usually very practical. Others have suggested taking an arithmetic average of several measurements to compensate for the so-called respiratory artifact. This averaging technique has not yielded satisfactory results. Still others have proposed ensemble averaging techniques which include superimposing a plurality of waveforms and obtaining information from a derived "average" waveform. But ensemble averaging techniques are not valid if the underlying signal is not strictly periodic.
In general, it is very difficult to analyze the signal representing the derivative of impedance. Various artifacts mask the critical features of this signal, and there has been no easy method of reliably extracting from the impedance signal the information needed for calculation of cardiac output.
The present invention provides an improved method and apparatus for measuring cardiac output, and overcomes the problems described above. The method of the present invention provides a reliable means of deriving meaningful data from bioimpedance measurements, while eliminating the effects of artifacts in the impedance signal.